CD28 is the major costimulatory molecule which in most circumstances is required for the efficient activation of antigen stimulated T cells. CTLA-4 is a second costimulatory molecule which shares considerable homology with CD28 but unlike CD28 it functions to attenuate the expansion of antigen activated T cells. Consistent with the role of CTLA-4, transient blockade of CTLA-4 function in vivo with anti-CTLA-4 monoclonal antibodies led to enhanced antitumor immunity in mice when used in conjunction with immunization protocols. Overall these studies have shown that CTLA-4 blockade could serve as a useful adjunct to antigen-specific immunotherapy to potentiate vaccine generated antitumor responses. In this grant application we propose to develop a new class of CTLA-4 inhibitors composed of short oligonucleotide aptamers. Aptamers are high affinity single stranded nucleic acid ligands which can be isolated trough a combinatorial chemistry process using iterative in vitro selection techniques termed SELEX. Importantly, most of these aptamers have been shown to be capable of inhibiting the function of the protein to which they bind. The goal of this grant application is to isolate aptamers that bind and inhibit human as well as murine CTLA-4 but not CD28. The specific aims of the proposal are: a) to isolate nuclease-resistant aptamers that specifically bind human CTLA-4 but not CD28 and which cross react with murine CTLA-4, b) to determine whether the CTLA-4 specific aptamers are capable of inhibiting the function of CTLA-4 in vitro and in vivo (in mice) and whether administration of the CTLA-4 aptamers to mice is associated with adverse effects and c) to explore methods to further improve the affinity and the in vivo bioavailability of the CTLA-4 aptamers. Successful completion of the proposed studies will set the stage for clinical trials to test the therapeutic benefit of using CTLA-4 binding aptamers in conjunction with immunotherapy in cancer patients.